The present invention relates to magnetic direction sensing systems and particularly those for use in vehicles.
U.S. Pat. No. 4,953,305, assigned to the present assignee, discloses a magnetic field sensor and microprocessor controlled compass system for a vehicle which is automatically and continuously calibrated to account for changes in the vehicle's magnetism and thus the system's reaction to the earth's magnetic field during the life of the vehicle. The system includes means for testing the data received from the compass sensor to determine the maximum and minimum signal levels during movement of the vehicle through a completed 360.degree. path of travel however circuitous the path may be. This data is averaged over several such paths of vehicular travel to provide continuously updated and averaged compensation correction information.
The compass system of the above-mentioned patent senses the magnitude of the earth's magnetic field in two channels of measurement. This data, converted into count values, can be plotted on an X-Y coordinate plane and, when the compass system is properly calibrated, creates a perfect circle around the origin of the plane when the vehicle has completed a 360.degree. path of travel. The radius of the circle represents the earth's magnetic field strength, and the vehicle's compass heading at a particular time during travel is represented by a point on the circle. By calculating the angle which the point forms with the X-Y coordinate plane, the compass heading of the vehicle may be determined.
Although this system is a substantial improvement in vehicle compass operation and provides more accurate heading information over differing operating conditions, it is subject to display error in environments in which the earth's magnetic field is largely vertical in nature. The earth's magnetic field, comprised of both vertical and horizontal components, is mostly horizontal near the equator and progressively becomes more vertical as one travels into the northern or southern latitudes. When the earth's magnetic field is predominantly vertical, the compass sensor, which is oriented to sense the horizontal components of the earth's magnetic field, senses a smaller than usual magnetic field. As a result, the plot of sensor data (converted into count values) creates a smaller than usual circle when the vehicle travels in a closed loop. Because a smaller circle is comprised of fewer count values than a larger circle, a smaller circle can be resolved into fewer angles from which the heading of the vehicle is calculated. This decreased resolution causes the accuracy of the heading information provided by the compass system to be less than optimal under such environmental conditions.
Thus, there exists a need for a compass compensation system capable of adapting to environments having relatively low sensed magnetic field strengths so that highly accurate heading information may be provided under such conditions.